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CSS
Cascading Style Sheets (CSS) is a used for describing the of a document written in a like . CSS is a cornerstone technology of the , alongside HTML and . CSS is designed to enable the separation of presentation and content, including , s, and . This separation can improve content , provide more flexibility and control in the specification of presentation characteristics, enable multiple s to share formatting by specifying the relevant CSS in a separate .css file, and reduce complexity and repetition in the structural content. Separation of formatting and content also makes it feasible to present the same markup page in different styles for different rendering methods, such as on-screen, in print, by voice (via speech-based browser or ), and on tactile devices. CSS also has rules for alternate formatting if the content is accessed on a . The name cascading comes from the specified priority scheme to determine which style rule applies if more than one rule matches a particular element. This cascading priority scheme is predictable. The CSS specifications are maintained by the (W3C). Internet media type ( ) text/css is registered for use with CSS by RFC 2318 (March 1998). The W3C operates a free for CSS documents. In addition to HTML, other markup languages support the use of CSS including , , , and . Syntax CSS has a simple and uses a number of English keywords to specify the names of various style properties. A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors, and a declaration block. Selector In CSS, selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself. Selectors may apply to the following: *all of a specific type, e.g. the second-level headers *elements specified by , in particular: **''id'': an identifier unique within the document **''class'': an identifier that can annotate multiple elements in a document *elements depending on how they are placed relative to others in the . Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens and underscores. A class may apply to any number of instances of any elements. An ID may only be applied to a single element. Pseudo-classes are used in CSS selectors to permit formatting based on information that is not contained in the document tree. One example of a widely used pseudo-class is , which identifies content only when the user “points to” the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in or . A pseudo-class classifies document elements, such as or , whereas a pseudo-element makes a selection that may consist of partial elements, such as or . Selectors may be combined in many ways to achieve great specificity and flexibility. Multiple selectors may be joined in a spaced list to specify elements by location, element type, id, class, or any combination thereof. The order of the selectors is important. For example, div .myClass {color: red;} applies to all elements of class myClass that are inside div elements, whereas .myClass div {color: red;} applies to all div elements that are in elements of class myClass. The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it. Declaration block A declaration block consists of a list of declarations in braces. Each declaration itself consists of a property, a colon (:), and a value. If there are multiple declarations in a block, a semi-colon (;) must be inserted to separate each declaration. Properties are specified in the CSS standard. Each property has a set of possible values. Some properties can affect any type of element, and others apply only to particular groups of elements. Values may be keywords, such as "center" or "inherit", or numerical values, such as (200 pixels), (50 percent of the viewport width) or (80 percent of the window width). Color values can be specified with keywords (e.g. " }}"), hexadecimal values (e.g. }}, also abbreviated as }}), RGB values on a 0 to 255 scale (e.g. ), RGBA values that specify both color and alpha transparency (e.g. ), or HSL or HSLA values (e.g. , ). Length units Non-zero numeric values representing linear measures must include a length unit, which is either an alphabetic code or abbreviation, as in 200px or 50vw; or a percentage sign, as in 80%. Some units – cm ( ); in ( ); mm ( ); pc ( ); and pt ( ) – are absolute, which means that the rendered dimension does not depend upon the structure of the page; others – em ( ); ex ( ) and px ( ) – are relative, which means that factors such as the font size of a parent element can affect the rendered measurement. These eight units were a feature of CSS 1 and retained in all subsequent revisions. The proposed CSS Values and Units Module Level 3 will, if adopted as a W3C Recommendation, provide seven further length units: ch; Q; rem; vh; vmax; vmin; and vw. Use Before CSS, nearly all presentational attributes of HTML documents were contained within the HTML markup. All font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML. CSS lets authors move much of that information to another file, the style sheet, resulting in considerably simpler HTML. For example, headings (h1 elements), sub-headings (h2), sub-sub-headings (h3), etc., are defined structurally using HTML. In print and on the screen, choice of , , and for these elements is presentational. Before CSS, document authors who wanted to assign such characteristics to, say, all h2 headings had to repeat HTML presentational markup for each occurrence of that heading type. This made documents more complex, larger, and more error-prone and difficult to maintain. CSS allows the separation of presentation from structure. CSS can define color, font, text alignment, size, borders, spacing, layout and many other typographic characteristics, and can do so independently for on-screen and printed views. CSS also defines non-visual styles, such as reading speed and emphasis for aural text readers. The has now the use of all presentational HTML markup. For example, under pre-CSS HTML, a heading element defined with red text would be written as: Chapter 1. Using CSS, the same element can be coded using style properties instead of HTML presentational attributes: Chapter 1. The advantages of this may not be immediately clear (since the second form is actually more verbose), but the power of CSS becomes more apparent when the style properties are placed in an internal style element or, even better, an external CSS file. For example, suppose the document contains the style element: All h1 elements in the document will then automatically become red without requiring any explicit code. If the author later wanted to make h1 elements blue instead, this could be done by changing the style element to: rather than by laboriously going through the document and changing the color for each individual h1 element. The styles can also be placed in an external CSS file, as described below, and loaded using syntax similar to: This further decouples the styling from the HTML document, and makes it possible to restyle multiple documents by simply editing a shared external CSS file. Sources CSS information can be provided from various sources. These sources can be the web browser, the user and the author. The information from the author can be further classified into inline, media type, importance, selector specificity, rule order, inheritance and property definition. CSS style information can be in a separate document or it can be embedded into an HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so that authors can tailor the presentation appropriately for each medium. The style sheet with the highest priority controls the content display. Declarations not set in the highest priority source are passed on to a source of lower priority, such as the user agent style. The process is called cascading. One of the goals of CSS is to allow users greater control over presentation. Someone who finds red italic headings difficult to read may apply a different style sheet. Depending on the browser and the web site, a user may choose from various style sheets provided by the designers, or may remove all added styles and view the site using the browser's default styling, or may override just the red italic heading style without altering other attributes. Specificity Specificity refers to the relative weights of various rules. It determines which styles apply to an element when more than one rule could apply. Based on specification, a simple selector (e.g. H1) has a specificity of 1, class selectors have a specificity of 1,0, and ID selectors a specificity of 1,0,0. Because the specificity values do not carry over as in the decimal system, commas are used to separate the "digits" (a CSS rule having 11 elements and 11 classes would have a specificity of 11,11, not 121). Thus the following rules selectors result in the indicated specificity: Example Consider this HTML fragment: To demonstrate specificity In the above example, the declaration in the style attribute overrides the one in the